In conventional analogue modulation techniques a signal is used to modulate one or more of the parameters of a carrier wave i.e. phase, amplitude or frequency. The carrier wave is usually of a frequency significantly greater than that of the modulating signal. As a result the modulated parameter may be assumed to be constant over several cycles of the carrier wave. Conventional demodulation techniques, such as zero-crossing counting in the case of frequency modulation, make use of this to obtain accuracy by averaging over several carrier signal cycles.
In applying digital signal processing techniques to demodulation the methods used to date have in general been functionally equivalent to the standard analogue techniques. Examples of such techniques are described in GB-A-2144288; EP-A-0046213; and, US-A-4567442. Typically the carrier is translated to an intermediate frequency by means of a superheterodyne process but this intermediate frequency still has to be high enough for the assumption that the modulated parameter is constant to hold for most demodulation techniques. In the case of frequency modulation, for example, digital zero-crossing techniques have been used in which the frequency at which the numerical representation of the signal falls below a threshold value is assessed over several cycles. Such techniques require a sampling rate high enough to produce an accurate digital representation of the modulated intermediate frequency signal since lower sampling rates result in unacceptable levels of distortion caused by aliasing. Therefore even though digitisation is carried out after translation of the carrier to the intermediate frequency it is still necessary to use a high sampling rate at least as great as the Nyquist rate characteristic of the intermediate frequency. The need to sample at a high rate and to transmit and process the resulting large bandwidth digital signal, a substantial part of which is redundant since all the information is contained within the narrower bandwidth characteristic of the modulating signal, is in many applications a significant disadvantage.